This invention relates to cardiac pacemakers, and more particularly to implantable programmable cardiac pacemakers that respond to the occurrence of a premature ventricular contraction (PVC) in a prescribed manner. Specifically, a pacemaker in accordance with the present invention minimizes the likelihood that a PVC response is inappropriately triggered. Moreover, when a PVC response is provided, it is designed to reduce the likelihood that a pacemaker mediated tachycardia (PMT) will be triggered, or that the pacemaker will otherwise be prevented from efficiently performing its intended function.
In order to efficiently perform its function of a pump, the heart must maintain a natural AV synchrony. The term "AV synchrony" relates to the sequential timing relationship that exists between the contractions of the atria and the ventricles. In a given heart cycle or beat, the atria (A) contract prior to the ventricles (V) in accordance with a prescribed timing or synchronized relationship, hence the term "AV synchrony." These contractions are typically manifest or measured by sensing electrical signals or waves that are attendant with the depolarization of heart tissue, which depolarization immediately precedes (and for most purposes can be considered concurrent with) the contraction of the cardiac tissue. These signals or waves can be viewed on an electrocardiogram (ECG) and include a P-wave, representing the depolarization of the atria; the QRS-wave (sometimes referred to as a R-wave, the predominant wave of the group), representing the depolarization of the ventricles; and the T-wave, representing the repolarization of the ventricles. (It is noted that the atria also are repolarized, but this atrial repolarization occurs at approximately the same time as the depolarization of the ventricles; and any electrical signal generated by atrial repolarization is generally minute and masked out by the much larger QRS-wave on the ECG.)
Thus, it is the P-QRS-T cycle of waves that represent the natural AV synchrony of the heart. These waves, including the timing relationships that exist therebetween, are carefully studied and monitored through conventional ECG techniques whenever the operation and performance of the heart is being examined.
A pacemaker is a medical device that assists the heart in maintaining a desired AV synchrony by producing stimulation pulses that are directed to an appropriate chamber of the heart in order to cause that chamber to depolarize, and hence contract. (Because the main function of the pacemaker is to provide such stimulation pulses, it is frequently referred to as a "pulse generator.") That is, if for some reason the heart is unable to maintain its natural AV synchrony, a pacemaker is utilized to monitor the heart and to provide electrical stimulation pulses when it senses the heart is not maintaining a proper AV synchrony. A dual chamber pacemaker, for example, monitors both the right atrium and right ventricle. If it senses an atrial depolarization at appropriate times, no atrial stimulation pulse is generated. If it senses a ventricular depolarization within a prescribed time after the atrial depolarization, no ventricular stimulation pulse is generated. If however, it fails to sense either the atrial or ventricular depolarization within prescribed time periods, then stimulation pulses, frequently referred to as an A-pulse (if delivered to the atrium) and a V-pulse (if delivered to the ventricle), are generated and delivered to the appropriate chamber of the heart at an appropriate time in order to maintain the correct heart rhythm.
A premature ventricular contraction (PVC) is a premature ventricular depolarization that results from an ectopic beat originating from one of the ventricles. A PVC is thus a ventricular event that occurs out of sequence, i.e., after a previous ventricular event without a prior intervening atrial event (P-wave or atrial pacer pulse). Thus, e.g., the natural AV synchrony of the heart may be represented by the sequence EQU P R P R P R P R P R . . . ,
where "P" signifies an atrial event (which could be either an A-pulse or a naturally occurring P-wave) and "R" indicates a ventricular event (which event can either be a V-pulse, or a naturally occurring R-wave). In contrast, the occurrence of a PVC might be represented, e.g., by the sequence EQU P R R P R P R P R R P R . . . ,
where the two consecutive R's indicate two ventricular events without an intervening atrial event. (It should be noted that the PVC sequence shown above is only one of several possible sequences that may result from the occurrence of a PVC. Indeed, as explained below, the problem with a PVC is that it can trigger so many different responses from a pacemaker, some of which responses are highly undesirable as they prevent the heart from resuming its natural AV synchrony.) The present invention is directed to a system and method of assuring that when a true PVC does occur, i.e., two consecutive ventricular events without an intervening atrial event, a pacemaker response results, if needed, that leads the heart back to its normal AV synchrony.
The normal sequence of events in a cardiac cycle begins with a stimulation pulse provided by the sinoatrial (SA) node of the right atrium, which stimulation pulse causes the atria to contract, i.e., causes the P-wave. (For this reason, the SA node is frequently referred to as the heart's natural pacemaker, as it sets the pace or rate at which the heart naturally beats.) As has been indicated, the P-wave is the result of the atria (both left and right atrium chambers) depolarizing and thus producing atrial contraction. When a dual chamber pacemaker is employed, this P-wave is detected by the pacemaker's sense amplifier via an atrial lead located in the atrial chamber of the heart. The P-wave depolarization then conducts through the AV node of the heart into the ventricles. As it does so, it is naturally delayed an appropriate amount, typically between 100 and 200 milliseconds, to allow the blood (being pushed from the atria by the contraction of the atria) to fill the ventricles. As the depolarization stimulus travels towards the ventricles, ventricular depolarization occurs. The QRS-wave is the result of this depolarization and thus represents ventricular contraction (which contraction pushes the blood from the ventricles to other parts of the body). This ventricular depolarization is also detected by the pacemaker's sense amplifier via a ventricular lead located in the ventricular chamber of the heart. It is sensed as an R-wave. When sensed, the pacemaker resets its timing circuits and inhibits the next scheduled pacer pulse to the ventricles. This cycle of events repeats when the SA node, after a period of recovery from the last depolarization, starts another cycle of a P-wave followed by an R-wave, and so on. If, at any point in the cycle, a P-wave or R-wave is not sensed, then the pacemaker provides an appropriate stimulation pulse in order to maintain the synchrony of the heart.
Unfortunately, a PVC does not follow the abovedescribed normal cycle of events because a ventricular event, i.e., an R-wave, is premature, happening before the next atrial event, e.g., P-wave. It is thus important for the pacemaker to reliably detect a PVC and to distinguish this event as unique. Otherwise, the pacemaker would respond by, e.g., resetting the timing circuits of the pacemaker in a manner that precludes sensing of a subsequent P-wave or R-wave, which action could cause A-pulses or V-pulses to be generated at inappropriate times, all of which could significantly disrupt the AV synchrony of the heart. Many of these problems resulting from the occurrence of a PVC in a patient with a dual chamber pacemaker are described more fully in applicant's prior patent, U.S. Pat. No. 4,788,980, which patent is incorporated herein by reference.
Recognizing this problem, it is common in the art for programmable dual chamber pacemakers to change the timing of the next cardiac cycle following the sensing of a PVC, which changed timing might include the extension of the atrial refractory period. (The atrial refractory period, as explained more fully below, is that period of time subsequent to the generation of a stimulation pulse or the sensing of depolarization during which no cardiac events are sensed.) This is done to avoid sensing retrograde conduction. Retrograde conduction, as explained in the referenced patent, is a condition where the depolarization of the ventricles propagates backwards into the atria, causing the atria to depolarize, which atrial depolarization in turn propagates through the AV node into the ventricles, causing the ventricles to depolarize I f retrograde conduction originating from a PVC continues over several cardiac cycles, a tachycardia may result. (A tachycardia is a very rapid rhythm or rate of the heart.)
Where a pacemaker is employed, operating in either the DDD or VDD modes, common operating modes for a patient having an abnormal AV conduction path, the pacemaker itself may cause the tachycardia by tracking each P-wave caused by the retrograde conduction, and providing a ventricular stimulation pulse a programmed P-V delay thereafter. The pacemaker thus provides the forward conduction path (from the atria to the ventricles) electronically by tracking each P-wave and generating a V-pulse (ventricular stimulation pulse) if no R-wave is sensed within a prescribed time thereafter (the programmed P-V delay). The reverse or backward conduction path (from the ventricles to the atria) is provided by retrograde conduction originating with the depolarization of the ventricles, which depolarization occurs as a result of the V-pulse. Thus, retrograde conduction passes the ventricular depolarization back to the atria, causing the atria to depolarize (resulting in a retrograde P-wave), and the process repeats. This is known as a Pacer Mediated Tachycardia, or PMT. Unfortunately, the occurrence of a single PVC can reset the pacemaker timing in a manner that allows the pacemaker to begin tracking retrograde P-waves, causing a PMT to occur. Hence, it is critically important that the pacemaker reliably sense a PVC and take appropriate action to prevent a PMT from being triggered.
One response to a PVC known in the art is for the pacemaker to revert to a DVI mode of operation for one cycle. (For an explanation of the various pacemaker modes, DDD, DDI, DVI, VVI, etc., see, e.g., U.S. Pat. No. 4,712,555.) This response, in effect, turns off the atrial sense amplifiers for one cycle. Thus, subsequent to the detection of the PVC, no P-waves can be sensed by the pacemaker because the electronic sense circuits are masked from sensing any atrial events, whether a retrograde event or a normal event. Hence, it is not possible for the pacemaker to generate a V-pulse one P-V delay after a retrograde P-wave, because the retrograde P-wave is not sensed, thus preventing a PMT. One problem with this approach is that if during the one cycle DVI response a normal sinus rhythm with spontaneous R-wave occurs, the PVC response remains on because the pacemaker interprets the spontaneous R-wave as another PVC, there having been no intervening atrial event (at least not one that was sensed, because the atrial sense circuits were off). Thus, even though a possible PMT is prevented, the loss of normal P-wave tracking may occur because P-waves are masked by the PVC response, and any R-wave is thus interpreted as a PVC. Hence, the PVC response may become "stuck", as there is no way for it to terminate. Loss of P-wave tracking may occur from seconds to hours depending on the pacemaker's programmed rate settings and the patient's sinus rate (i.e., the P-wave rate set by the SA node).
Another PVC response known in the art is to extend the Post Ventricular Atrial Refractory Period (PVARP) by a prescribed amount, such as 480 msec, thus masking retrograde conduction during this period of time. (This PVC response is referred to hereafter as the "+PVARP on PVC response," or simply a "+PVARP response.") In addition, the atrial escape interval (VA delay) is fixed to a prescribed value, such as 830 msec, regardless of the programmed or sensor indicated rate (if a sensor is used, such as is the case in a rate-responsive pacemaker). See U.S. Pat. No. 4,788,980. The difference between the selected PVARP value and the fixed VA delay, which difference is 350 msec for the example given, allows a P-wave to be detected. This approach is an improvement over the DVI on PVC approach described above because the extended PVARP interval is sufficient to mask most retrograde conduction in the majority of patients, and P-waves not related to retrograde conduction can still be tracked. However, unless the sinus P-wave or other atrial event (e.g., an A-pulse) occurs during the time period subsequent to the extended PVARP interval and prior to the termination of the VA delay (e.g., during the 350 msec time period for the example times given above), the PVC response continues. This can occur if P-waves fall within the extended PVARP interval (which will not be detected) followed by R-waves that cause the VA delay interval to be reset (with the R-waves being interpreted as PVCs). This PVC response will have a fixed atrial escape interval and will slow the ventricular rate down because the rate of pacing is made up of the AV delay and the atrial escape interval (VA delay).
Further, with the new technology of activity sensors used for rate responsive pacing, the pacing rate is controlled by a separate sensor that detects patient activity (or some other parameter indicative of the need to adjust the heart rate). If such an activity sensor is employed, and if the +PVARP response remains stuck in the PVC response, then the patient may suffer from a lower ventricular rate controlled by the PVC response and not by the sensor indicated rate, which may be at a much higher rate than the PVC response controlled rate. For example, if the AV delay is 150 msec, then the rate during a +PVARP on PVC response would be, using the same numbers presented above, 150 msec +830 msec =980 msec, or about 61 beats per minute. Any intrinsic P-waves falling within the +PVARP interval are not sensed, hence the +PVARP response remains on when accompanied by detected R-waves prior to the end of the VA delay, and thus prevents sensor controlled rates from being effective since the +PVARP response controls the atrial escape interval. In other word, since the +PVARP response slows the ventricular rate down from a higher sensor controlled rate, it is more likely that a sinus will keep the +PVARP response on, thereby causing the slower ventricular rate, which rate may be undesirable when the patient needs increased cardiac output during activity. Thus, the +PVARP response to a PVC may not be appropriate for a sensor driven rate responsive pacer.
In order to avoid a falsely detected PVC, particularly in a rate-responsive pacer, it is known in the art to shorten PVARP with increased sensor or P-wave activity. Such approach allows better P-wave tracking before a P-wave falls into the PVARP interval (i.e., it enlarges the time interval during which a P-wave may be sensed, which time interval is the difference between PVARP and the VA delay). Of course, once a P-wave falls into the PVARP interval, it is not tracked. Such an approach also reduces the likelihood of a P-wave occurring during PVARP followed by an R-wave being interpreted as a PVC, again because it is more likely that the P-wave will not occur during the shortened PVARP interval. Nonetheless, even with this approach it is possible to remain in a PVC response once the response has been initiated because the events that need to be sensed to terminate the PVC response, e.g. a P-wave, are not sensed, as when they occur during the PVARP interval, or are not sensed for other reasons (such as due to lead dislodgment).
Another possible response to a PVC is to pace the atrium on the sensing of a PVC. This response, known as "A PACE ON PVC", is a selectable feature on the PARAGON pacemaker available from Pacesetter Systems, Inc., of Sylmar, California. However, at present this feature is not used because it has the potential of causing atrial competition. That is, since this response issues an atrial pulse with every sensed PVC, if atrial sensing has been lost (such as might occur, e.g., through lead dislodgment) every R-wave is considered as a "PVC" by the pacemaker PVC detection circuits.
What is needed, therefore, is a system for responding only to true PVC's, i.e., two sequential cardiac ventricular events without an intervening cardiac atrial event, and that minimizes the likelihood that a pacemaker will interpret a non-PVC as a PVC because the pacer fails to sense the intervening atrial event. Further, once a true PVC is detected, a response is needed that minimizes the likelihood of triggering a PMT, and that guides the heart back to its normal AV synchrony. Finally, a PVC response system is needed that assures the PVC response will terminate when appropriate, i.e., that the PVC response will not become stuck. The present invention advantageously addresses these and other needs.